Hydrogen peroxide: a key messenger that modulates protein phosphorylation through cysteine oxidation.

Ligand-receptor interactions can generate the production of hydrogen peroxide (H(2)O(2)) in cells, the implications of which are becoming appreciated. Fluctuations in H(2)O(2) levels can affect the intracellular activity of key signaling components including protein kinases and protein phosphatases. Rhee et al. discuss recent findings on the role of H(2)O(2) in signal transduction. Specifically, H(2)O(2) appears to oxidize active site cysteines in phosphatases, thereby inactivating them. H(2)O(2) also can activate protein kinases; however, although the mechanism of activation for some kinases appears to be similar to that of phosphatase inactivation (cysteine oxidation), it is unclear how H(2)O(2) promotes increased activation of other kinases. Thus, the higher levels of intracellular phosphoproteins observed in cells most likely occur because of the concomitant inhibition of protein phosphatases and activation of protein kinases.